Tetrahydrobiopterin (BH4) is the rate-limiting cofactor for tyrosine and tryptophan hydroxylase, the initial enzymes which control the synthetic rates of the catecholamines and serotonin. Since aberrant metabolism of these neurotransmitters is thought to play a major role in a variety of neurological and psychiatric disorders, it was of interest to examine the disposition of BH4 in the mammalian brain. We previously reported that a specific neurochemical lesion with 6-hydroxydopamine in the nigrostriatal system of the rat produce an 85% reduction in reduced cofactor content which suggested a high degree of localization of BH4 to dopamine neurons in this system. We now report that total biopterin (B) and the activity of GTP-cyclohydrolase, the initial enzyme in BH4 synthesis, are depleted approximately 70% after this lesion whereas tyrosine hydroxylase (TH) activity is reduced by 90%. Kainic acid injection in the striatum which destroys cell bodies and interneurons while sparing nerve terminals caused a 30% reduction in B and 25% loss of TH. These results indicated that the majority of B and its biosynthetic system is localized to dopamine neurons of the nigrostriatal system. The nearly equivalent loss of B and TH after kainic acid is indicative of nonspecific aminergic terminal damage.